Apparatus comprising loudspeakers, which are configured to convert an electrical input signal to an acoustic output signal, are well known.
Such loudspeakers typically comprise a driver which is configured to drive the oscillation of a diaphragm to generate a pressure wave. The loudspeakers are often mounted within an acoustic cavity. An acoustic cavity prevents the pressure wave which is generated by the back of the diaphragm from destructively interfering with the pressure wave from the front of the diaphragm.
The volume of the acoustic cavity defines the resonant frequency of the loudspeaker. Acoustic cavities with a large effective volume have a lower resonant frequency than acoustic cavities with a small effective volume. The response of the loudspeaker decreases below the resonant frequency of the loudspeaker. This means that if the acoustic cavity has a small volume the loudspeaker may not be able to produce low frequency acoustic signals very well.
In some apparatus the volume available for an acoustic cavity is limited. For example in a handheld device such as a telephone or a personal music player only a small volume may be available for the acoustic cavity. This may result in poor sound quality for low frequency acoustic signals.